Stimulators, incubated with a suspension of normal, adult, rabbit marrow cells induced increases in basophilic erythroblasts, mitoses in acidophilic erythroblasts and reticulocytes. A similar stimulatory protein was obtained from rabbit leukocytes, erythrocytes, plasma and beef, pig and rabbit spleen. The protein from all the above sources (1) retained its activity after boiling at pH 5.0, (2) was precipitated in 20-40 per cent (NH4)2SO4, and (3) chromatographed the same on DEAE. Its activity destroyed by trypsin. Yield: about 20 micro-g/kg spleen. An EtOH extract of leukocytes was active. Its active principle was identified by chromatography and synthesis as lignoceric acid sphingosine ceramide; maximal response with 1 micro-g/ml. Palmitic and stearic acid sphingosine ceramides gave little or no stimulation. Some steroids of the pregnan, androstan and estrogen series were active, some inactive. Some stimulators of reticulo-endothelial cell and macrophage activity, tested because of ontogenic kinship to erythroblasts had stimulatory activity. CoCl2 and stilbestrol were active. Cholesterol, testosterone and heparin were inactive. All stimulators required erythropoietin and calcium and induced increases in the three above respects. All the inactive compounds were inactive in the three respects. The stimulators were inhibited completely by antibody to erythropoietin, and by inhibitors of oxidations, oxidative phosphorylation and of ribosomal function.